Flip chip microelectronic assembly is the direct, face-to-face electrical connection of an integrated circuit (“IC”) to a substrate or circuit board by means of conductive bumps (i.e., solder bumps) situated there between. More specifically, the IC is electrically connected to the substrate by way of the solder bumps during a solder reflow process, wherein the solder bumps are heated to cause the bumps to melt and establish an electrical connection between the IC and the substrate.
Substrates having solder bumps in relatively close proximity to each other are known as “fine pitch substrates.” A concern with fine pitch substrates is the possibility of one or more short circuits on a substrate due to the excessive flow of solder during a solder reflow process. For example, during a reflow process on a fine pitch substrate, two closely-situated solder bumps may melt and come into contact with each other, thus causing a short circuit and possibly rendering the IC and/or the substrate useless. Solder also may flow into other areas of the substrate, causing short circuits in or otherwise damaging these areas. These short circuits also may render the IC and/or the substrate useless.
Thus, fine pitch substrates may use solder masks to prevent short-circuiting between two solder bumps. Soldermasks also may be used to prevent solder from flowing into other portions of the substrate, as mentioned above. Furthermore, solder masks also may be used to protect various portions of the substrate during various package assembly steps (e.g., wet processes that may damage portions of the substrate). As shown in FIG. 1, a soldermask 10 having precisely-defined openings 20 is placed over a substrate 30. The openings 20 then may be filled with solder bumps affixed to the metal traces 15. Because the openings 20 are precisely-defined, solder flow is contained within the boundaries of the openings 20. Portions of the solder mask 10 not exposing metal traces 15 protect the substrate 30 from solder reflow and from damage during other portions of the assembly process (e.g., wet processes). However, because the position and size of the openings 20 in a soldermask 10 designed for fine pitch substrates may need to be precisely controlled, soldermasks for fine pitch substrates are expensive to manufacture.